Batteries (including non-rechargeable batteries and rechargeable batteries) have wide applications. For example, batteries are employed in electronic devices, such as mobile phones, laptop computers, and portable medical devices. Batteries are also employed in automobiles, such as gasoline or diesel powered vehicles, hybrid vehicles such as electric-gasoline powered hybrid vehicles, and purely electric vehicles.
For some battery applications, it is important to provide accurate information to users or technicians about the capacity of the battery. The capacity of the battery indicates how much charge the battery still holds (e.g., the remaining capacity), has lost (e.g., the degradation capacity), or has been discharged (e.g., the discharge capacity). Thus, the term “capacity” may refer to the discharge capacity, the degradation capacity, or the remaining capacity. It is understood that the remaining capacity may be calculated from the discharge capacity and vice versa. For example, the remaining capacity may be calculated by subtracting the discharge capacity from a maximum capacity of the battery, and the discharge capacity may be calculated by subtracting the remaining capacity from the maximum capacity. Discussions below refer to the estimation of the discharge capacity. It is understood that once the discharge capacity is estimated, the remaining capacity may be estimated from the estimated discharge capacity based on the above described relation.
The capacity of the battery may be represented using units such as watt-hours (Wh), ampere-hours (Ah), or coulombs. The capacity of the battery may also be represented by state of charge (SOC). The SOC as used herein refers to a percentage, which varies between 0% (fully discharged state) and 100% (fully charged state). The percentage representing the SOC indicates the remaining capacity relative to the full capacity of the battery at the current status. The full capacity may be equal to or closer to a nominal maximum capacity when the battery is new, and may become lower than the nominal maximum capacity after the battery has been used or after the battery has degraded. For example, when a battery is new, the nominal maximum capacity may be 7.2 Ah. Under this condition, 100% SOC means the battery capacity is 7.2 Ah, and 50% SOC means the battery capacity is 3.6 Ah. After the battery has been used, the maximum capacity may have dropped to 6.0 Ah, lower than the nominal maximum capacity. Under this condition, 100% SOC means the battery capacity is 6.0 Ah, and 50% SOC means the battery capacity is 3.0 Ah. Thus, under two different conditions, the same percentage (e.g., 50%) SOC may mean different capacities. Accordingly, using SOC for representing the remaining capacity may be misleading to users in some applications, e.g., when the users need to know how much absolute capacity remains.
In some applications, the remaining battery capacity may also be represented by an absolute state of charge (ASOC). The absolute state of charge shows the remaining capacity relative to the nominal maximum capacity when the battery is new. For example, when the battery is new, if the nominal maximum capacity is 7.2 Ah, then at any time, a 50% ASOC indicates that the battery has a capacity of 3.6 Ah remaining.
To determine the capacity of a battery, such as a rechargeable battery, some current methods utilize a look-up table that stores data relating to a voltage and a capacity of the battery. The look-up table may store data pairs of (open-circuit voltage, remaining capacity) or (open-circuit voltage, discharge capacity). Once an open-circuit voltage is measured at any given state, the capacity of the battery may be estimated from the look-up table. The data may be derived from a voltage-capacity characteristic curve of the battery. For convenience of description, the characteristic curves, rather than look-up tables, are often directly referred to in the description below.
FIG. 1 shows exemplary voltage-capacity characteristic curves of a battery. The curves shown in FIG. 1 represent a relationship between the open-circuit voltage (OCV) (vertical axis) and the discharge capacity (horizontal axis). It is understood that characteristic curves representing a relationship between the OCV and the remaining capacity may be obtained since the remaining capacity can be calculated from (maximum capacity−discharge capacity).
FIG. 1 illustrates a charge-mode characteristic curve 100 and a discharge-mode characteristic curve 110. The unit for the discharge capacity is ampere-hours (Ah). Other units, such as watt-hours (Wh) or coulombs may also be used to represent the capacity of the battery. For descriptive purposes, it is assumed the nominal maximum capacity of the battery is M1 Ah when the battery is new. The point corresponding to 0 Ah on the horizontal axis means zero charge has been discharged (e.g., used or lost), and thus, the battery is in a fully charged state with a remaining capacity of M1 Ah. The number 1.8 Ah on the horizontal axis means 1.8 Ah has been discharged (e.g., used), and thus, the remaining capacity of the battery is (M1−1.8) Ah. Given any discharge capacity D1 on the horizontal axis, the remaining capacity can be calculated as (M1−D1) Ah. The maximum capacity for a battery may depart from the nominal maximum capacity after the battery has been used. For example, due to degradation, the maximum capacity may drop from M1 Ah to (0.5*M1) Ah.
Still referring to FIG. 1, the vertical axis represents the open circuit voltage (OCV) of the battery. The open circuit voltage is measured when the battery is at an idling state. The idling state is defined as a state in which no load is applied to the electric terminals of the battery, and the battery has been at rest for more than a certain amount of time, for example, 20 minutes. When measuring the OCV, only the measuring device or circuit is connected to the electric terminals of the battery.
The terms “charge-mode” and “discharge-mode,” as used in the terms “charge-mode characteristic curve” and “discharge-mode characteristic curve” do not mean that the OCV is measured when the battery is being charged (e.g., in a charging process) or being discharged (e.g., in a discharging process), because the OCV is measured in an idling state. Instead, the “charge-mode” or “discharge-mode” only means that under the current state, behavior of the battery may be described according to the charge-mode characteristic curve or the discharge-mode characteristic curve.
As illustrated in FIG. 1, each point on either the charge-mode characteristic curve 100 or the discharge-mode characteristic curve 110 corresponds to a value indicating the OCV and a value indicating the discharge capacity. Look-up tables may be provided for storing data pairs of the corresponding characteristic curve, and may be used to determine the capacity once the OCV is known. However, there may be problems associated with methods using a look-up table in estimating the capacity (e.g., the remaining capacity or the discharge capacity). For example, after the battery has been used, the battery may degrade. As a result, the maximum capacity of the battery may have dropped. In addition, the OCV-capacity relationship may change over time. Therefore, the original look-up table may not reflect the actual relationship between the OCV and the capacity. Thus, the original look-up table may not provide an accurate estimate of the capacity.
Another phenomenon that may cause problems is that the battery may exhibit two different curves, such as the charge-mode characteristic curve 100 and the discharge-mode characteristic curve 110, as shown in FIG. 1. For some batteries, the charge-mode characteristic curve and the discharge-mode characteristic curve may be identical, substantially overlap each other, or having insignificant differences, such that the two curves can be treated as a single curve without causing significant errors in estimating the remaining capacity. However, for some batteries, for example, LiFePO4 type batteries, the charge-mode characteristic curve and the discharge-mode characteristic curve may exhibit substantial differences such that they may not be treated as a single curve for accurately estimating the capacity.
For example, in FIG. 1, OCV=3.3 V corresponds to two different discharge capacities, one being about 0.6 Ah, the other being about 1.3 Ah. Thus, the discharge (and the remaining) capacities determined based on these two OCV's have a difference of about 0.7 Ah, which may be significant in some applications. This problem may occur when it cannot be determined with certainty which one of the charge-mode characteristic curve 100 and the discharge-mode characteristic curve 110 (and which corresponding look-up table) should be applied to estimate the capacity.
Another phenomenon that may also cause problems in using look-up tables to estimate the capacity is illustrated in FIG. 2. When a battery (e.g., a LiFePO4 type battery) transitions from a first state 105 where the discharge-mode characteristic curve 110 may be suitable for estimating the capacity, to a second state 115 where the charge-mode characteristic curve 100 may be suitable for estimating the remaining capacity, the battery may need to receive a certain amount of charge Q to complete the transition. This transition may take some time. During the transition, the battery's state may follow a route 120 shown in FIG. 2 connecting the first state 105 to the second state 115. Thus, when the OCV is measured to be 3.3 V as the battery state is following the route 120, the route 120, rather than the characteristic curves 100 and 110, should be considered to determine the capacity. Using either the charge-mode characteristic curve 100 or the discharge-mode characteristic curve 110 to estimate the capacity when the battery is in the transition state will likely cause errors in the estimate.
A further phenomenon that may cause errors in estimating the capacity using the characteristic curves and their corresponding look-up tables is that for some batteries, such as LiFePO4 type batteries, the slope of the characteristic curves (OCV versus discharge capacity or OCV versus remaining capacity) tend to be relatively flat in a middle section, as shown in the exemplary OCV versus discharge capacity curve in FIG. 3. Thus, a small deviation in the measured OCV value within the middle section may nevertheless cause a large error in the estimated capacity. For example, when the battery has not reached the idling state (e.g., the battery has not been at rest for a sufficient amount of time), measuring the OCV of the battery may result in an error in the measured OCV, which may in turn lead to a significant error in the estimated capacity due to the relatively flat slope.